HI Amol,

I dont think you can generate dynamic SOQL from your process-conf file, reason being you cannot call Salesforce from within that file.
Nagendra provided Apex code, but I dont know how that would work in this scenario, please do let me know Nagendra.

I purpose you utilise Ant Migration tool's ability to query the object's metadata and make a ant target to parse the .object file and generate the process-conf from your ant build.xml
Below is an example on how would I approach this problem:

<?xml version="1.0" encoding="UTF-8"?>
<project name="Export" default="backup" xmlns:sf="antlib:com.salesforce">
	<taskdef resource="net/sf/antcontrib/antlib.xml"/>
    <property file="build.properties"/>
    <property environment="env"/>

    <macrodef name="export">
        <attribute name="file"/>
        <attribute name="object"/>
        <attribute name="soql"/>
        <attribute name="userName"/>
        <attribute name="password"/>
        <attribute name="serverURL"/>
        <attribute name="batchSize"/>
        <attribute name="limit"/> 

        <sequential>
            <echo message="Exporting @{object}"/>
            <mkdir dir="exports/DATE"/>
            <mkdir dir="logs/DATE"/>
            <copy file="config/template-process-conf.xml" tofile="config/process-conf.xml" overwrite="true" failonerror="true"/>
            <replace file="config/process-conf.xml">
                <replacefilter token="_object_" value="@{object}"/>
                <replacefilter token="_soql_" value="@{soql}"/>
                <replacefilter token="_file_" value="exports/${todayDate_only}/@{file}.csv"/>
                <replacefilter token="_serverURL_" value="@{serverURL}"/>
                <replacefilter token="_userName_" value="@{username}"/>
                <replacefilter token="_password_" value="@{password}"/>
                <replacefilter token="_batchSize_" value="@{batchSize}"/>
                <replacefilter token="_logFile_" value="logs/DATE/@{file}_log.csv"/>
            </replace>
            <java classname="com.salesforce.dataloader.process.ProcessRunner" classpath="C:\YOUR_PATH\dataloader-39.0.0-uber.jar" failonerror="true">
                <sysproperty key="salesforce.config.dir" value="config"/>
                <arg line="process.name=@{object}"/>
            </java>
        </sequential>
    </macrodef>
	
	<!--Step 1-->
	<target name="retrieveObjects">
		<mkdir dir="retrieveObjects"/>
		<sf:retrieve 
			userName="${sf.prod.profileName.username}"
			password="${sf.prod.profileName.passwordUnencrypted}"
			serverURL="${sfProd.serverurl}"
			retrieveTarget="retrieveObjects"
			unpackaged="retrieveSource/package.xml"/>
	</target>
	
	<!--Step 2-->
	<target name="foo">
		<foreach target="backup" param="file">	
			<path>
				<fileset dir="YOUR_PATH_objects" includes="**/*.object"/>
			</path>
		</foreach>
	</target>
	
	<target name="backup">
		<xmlproperty file="retrieveObjects/objects/Account.object" collapseAttributes="true"/> <!--this parses the xml file-->

		<export
            file="Account_backup-DATE"
            object="Account"
            soql="select ${CustomObject.fields.fullName} from Account"
            userName="${sf.prod.profileName.username}"
            password="${sf.prod.profileName.password}"
            serverURL="${sfProd.serverurl}"
            batchSize="200"
            limit = "1000" />
    </target>
</project>

for this to work you need:

• ant migration tool properly configured​
• download and install ant-contrib for the foreach loop to work
• make a template-process-conf.xml and copy this code into it, and modify it as you need

This ant build.xml use a template file to generate the actual process-conf with replaced variables. Give this a try and let me know if you get any lucks or questions.