I'm not convinced that practicing finger exercises is necessary for playing traditional music on tenor banjo. The tunes themselves are the etudes, and if you practice tunes slowly and mindfully as you are learning them you incorporate picking exercises into your routine by default.
Still, every once in a while it's helpful to focus on scales and exercises specifically designed to improve technique and increase speed and accuracy. Enda Scahill's Irish Tenor Banjo Tutors, Volumes I and II, are a great source for this type of instruction. Below are two examples of exercises (out of many) from his Irish Banjo Tutor Volume II.
According to Enda, practicing these exercises will stretch out the fingers, improve their separation and independence from each other and strengthen the fingers. I highly suggest you purchase his tutors if you do not have them already.