I am kind of in the same situation, I am a java guydeveloper but now my employers/coworkers expect me to do some cobol/batch support work since a senior employee who handled these stuff recently quit and they are struggling to get things going. I have no interest in learning an older/outdated technology like cobol which I never plan on adding to my resume. So I always push back when they jokingly make invites to me to join the meetings for cobol related issues. Also, there is no pay raise for this offer and I am not taking up extra responsibility and let the owners pocket the profit.
In your case, you should decide for yourself if you are willing to spent your time for no monetary gain and probably not much career progress as well with the new responsibility. If not, start looking for a better opportunity somewhere else. If you do choose to stay, try to get a college degree(if you don't have one already) in IT field on the side, so while you wait and gain some experience and ready to move on, the degree will help you get through the door.