As I suggested, if you don't know when the fluid was last replaced, consider it in need of replacement. Adding fluid in the short term will not damage the transmission. If you don't know what type of fluid is in the transmission, you would be better off replacing it, sooner rather than later.
Do you have a manual gearbox or an automatic transmission? Neither should be low on fluid unless there's a leak, which would be my first job to tackle.
Yes, you can top off the fluid, whether it's gear oil or ATF, with new. If you don't know when it was changed last, assume it needs a change...