No, if you're on days off, you're on days off, simple.
There's no such thing as reserve. Standby duties are written into your schedule and there's probably about a 30% chance of you being used (they normally change the standby to a flight before the standby day arrives). These are normally one or two days in a row, although lately there have been examples of more consecutive days.
There are guys here trying to fight to preserve our days off and the flexibility of being able to chose when we want to take those days. A pilot who wants to prostitute himself and screw around volunteering to waive his days off all in the name of money will not be well received and will earn himself a bad reputation.....not good to be an overtime whore in such a small company.