If you already have some shellac then that will be fine, what is the point of buying extra sealant for one pen.
As Grandma use to say, anything worth doing is worth doing right. If you cut corners, then the next thing you will hear is, "I don't have shellac or thread sealant, can I use tile chalk? Also all joking around aside, think about 20 years from now when the pen needs serviced by someone else and what has to be done to keep the pen in good working order. I have seen some really screwed up repairs jobs, and I have seen some pens ruined because of it.