Theoretically easily with iMacros. Record the following process and specify your list as data source, one username per line, the list can be .txt or .csv. You need to play the macro in loop, the loop count equals to the number of users you want to follow. Sounds easy enough, but in practice you'll most certainly face rate limits and account locks, so you need to insert high enough random delays between two follow actions. This can also be done with iMacros, which has an extensive wiki, so you shouldn't have any issue making the macro.