Indeed you probably can. You may well find the boiler has specific terminals for a stat that are distinct from those for the programmer, however the chances are they are just wired in series anyway.
programmable stat like:
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way you can have the house at different temperatures throughout the day without needing to adjust the stat all the time. It also lets you set minimum temperature limits more easily. So for example, rather than having a timer simply turn the heating off overnight, you could have the stat fallback to maintaining the temperature above 15 degrees say. That way the heating will not usually fire at night unless you have a really cold night when it will make it easier to get the place back to a comfortable temperature in the morning.
(you can get more and less sophisticated types of prog stats - the one linked to allows 7 day programming, simpler ones only allow 24 hours, and posher ones are "optimising" stats that learn the heating characteristics of the house)