Inconsistent data will drive you crazy. And there is no magic cure. If the data was strictly numeric, you can put the data in a field of type number, but since you have indicated that there is additional text that can be in this match field along with the numeric digits, this may not be possible to do.
Ultimately, you need internally generated values to use as your primary keys for linking to other tables and these fields of imported data should not be used for that purpose, but immediately after import, you may still have to use this data as best you can to find and, if needed, generate the needed interal match value.
If you really needed to link the two files, you could use a calculation to add "0"' as fillers.
Case ( Length ( Key) = 1 ; "00" & Key ; Length ( Key ) = 2 ; "0" & Key ; Key )
Just make sure the field is a text field.