When we enter a new Lead into Salesforce we have a lookup field to add the Campaign rather than having to add the Lead then go to the add Camaign separately. Once the Lead has been saved the Campaign lookup is no longer visible in the Lead page layout.  Sometimes the Campaign is not known at the time of adding the Lead, however we do need to know the Campaign prior to the Lead being converted.  This can be done by adding the Campaign as normal, however our sales people do not always remember to do this. 

I looked at maybe making it a required field on the Converting page however you cannot change the converting page layout.  I have been advised by Salesforce Support to create a Validation Rule to make the Campaign a required field upon conversion, however they do not support validation rules.  I am not sure how to do this.  Can anyone help with how to make the Campaign lookup field, that is visible in the entering of a lead stage, a required field on converting?

There may also be another way to do this.  I am open to suggestions, I just need to make sure that there is an associated Campaign prior to the lead being converted without having to rely on the user to enter it.

Thank you.