I tried the Source Substring checkbox - it truncated the longer values to 22 char, but also it padded all of the blank fields with 22 spaces.
The Export Substring checkbox didn't appear to do anything.

I think I got it, 

set Operation to Calculated
Calculate MID(AR_InvoiceHistoryHeader01.CustomerPONo$,1,22)

Well done Mike. Since you mentioned blank spaces, in case you also want to strip the trailing spaces from the 1st 22, then:

STP(MID(AR_InvoiceHistoryHeader01.CustomerPONo$,1,22))

Also just so you're aware, it's fairly common to pull data from Sage (e.g. Access, Excel, etc) as an alternative to pushing data from V/I Exports. The best solution just depends on your project needs.