Simply put the following formula into a formula field of type "Text":

Id
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,1,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,2,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,3,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,4,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,5,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,6,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,7,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,8,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,9,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,10,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,11,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,12,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,13,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,14,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,15,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)

This is an optimized version of the code example "BPS: 18 digit external ID within the UI" you can find on the Online Help, but unlike the example provided there, it is small enough to fit into a formula field. Also, this code doesn't contain the error found in the online help example, where the second character byte equivalent of the first 5-character chunk was "4" instead of "2". I'd be more than happy if someone could come up with an even shorter version of this formula...Still, I hope you can use this.

Cheers,

Erik Mittmeyer