Solo workers tend to be fine with this if time permits.
Brothels tend to want to charge you the full whack of the extra time you want.
Massage parlours might let you top up if they're nice and/or you're a regular.
No harm in asking. I do, if I'm not convinced of the reputation of the place or the quality of service from the girl. If they say you have to pay the full price for the additional time I usually take that as a bad sign and I'm more likely to walk if the girl herself is less than charming when we meet.