Open ICDDEFS7.BAS

This one reserves space for ICD vars for the 18F2220/2320/4220/4320.

ICDRESERVED12 VAR BYTE(12) $1f4

Now open your ICD2 help file, and look under "Resources Used By MPLAB ICD 2".

Locate the 18F2220, and look under the "File Registers Used" column. See where it
shows 0x1F4-0x1FF?

So all the above is doing is reserving 12 bytes of RAM from 0x1F4-0x1FF.

Now scroll down a bit and look for the RAM space you need to reserve for the 2550
and 4550. Now create your new file with ICDRESERVED12 VAR BYTE(12) followed
by the start address shown in the help file.

I don't use ICD2 so I can't offer much help beyond this point, but that's all they appear
to be including in these files.