When you have numbered your Sosa, if you had chosen to keep in your GECDCOM, this will create in your entities a special tag that you can use to filter the entities you keep in your new file.
When you "save as" new file, choose the "Filter by entities" tab and check "Only include individuals with following tag" and enter the sosa tag.
_SOSA if you used Sosa numbering
_DABOVILLE if you use the d'Aboville numbering
_SOSADABOVILLE if you use the special Sosa/ d'Aboville numbering
Zurga