The thing about it is you would be giving your user too much control of the database, this is very bad. Can this be done? Yep, quite easily actually, should it be done? No way. What you could do is have them add up to 4 new fields, and have these fields as blank options in your current table. This way they aren't adding to your database, but filling in some optional room you made for them. Also, make sure you have headings for these stored somewhere as well, which you can make an additional field for that.

Hope that helps