If you are talking about a "symbol" then sure you could place one on a weapon but in general it would do no good since you have to be able to "read" the symbol in most cases in order for it to go off. In general you can't read anything on a weapon while in combat.
Rune magic is from the Forgotten Realms Campaign Setting book. It allows you to create runes that do various effects.
In it, it says you can place a rune on any medium object, but only one. Didn't say anything about applying a rune to and already magical weapon. I was asking if anyone knew if there was more explained about it somewhere.