You might consider using auto-enter calculations that does a character substitution as part of the import:
Substitute ( Self ; [ "-" ; "**"] ; ["20**" ; "" ] )
But maybe 20** is using the ** as wild cards? If so, then scratch this suggestion
and if not, note that which substitution you do first could affect your results since the first replace adds ** that might then get cleared by the second replace.
"Replace Field Contents" is a choice work on PSOS.
For attention, this can't change value if the imported found set have records locked by other user at the time.
OTOH, auto-enter works through importing, but not only the time, further editting the field also fire it. You can change the behavior using condition something like
Case ( Get ( some condition flag ) = <<some condition>> ; <<calc>> ; Self )