If the field on the form is for display only (i.e. not for data entry), and if your data is consistent, e.g. always six digits, you could use a calculated field with a formula like this:
iif(size([:NEWBLE:MEMBERS.YNumber]) = 6, subStr([:NEWBLE:MEMBERS.YNumber],1,3) + "-" + subStr([:NEWBLE:MEMBERS.YNumber],4,3), [:NEWBLE:MEMBERS.YNumber])
In the above case, the field, YNumber, is in a table ":NEWBLE:MEMBERS.DB
If you need to have the field for data entry, then I think you will need some tricky enough code, and it may be easier to maintain 2 fields - one for what you want to have displayed/entered (use a picture) and one for "internal" use. When a record gets written/updated, use code to update the "internal" field.
HTH
Padraig