Depending on the modpack, you can create a "machine" called Energy bridge. That can be used in conjunction with BC (Buildcraft, meaning MJ) consumer and IC2 (IndustrialCraft2, meaning EU) producer. After you place them both adjacent to the Energy bridge, you can input MJ into the BC consumer and it outputs EU from the IC2 producer.
Please note that there are 4 "tiers" of IC2 consumers/producers (LV < MV < HV < EV), and BatBox can only handle 32 eu/p, which means LV.