My employer offers a defined benefit pension plan, but allows the employee to do additional contributions. Now, if I contribute to my employer pension, does that take away from the RRSP contribution room that I have?
Yes it does. The amount of tax sheltered pension room is fixed every year. You either use it for your RPP or RRSP. Note that the employer's contributions are also accounted for in your T4 in box 52 as pension adjustment and reduce your contribution room.
You will have to read your pension rules to get the specifics. You may be talking about past service buybacks or something completely different.
Unless your employer's pension plan has RRSP matching, I doubt it's going to be under a registered account. In short, it's unlikely that would take away your RRSP contribution room, but do clarify with the company because policies vary.