Apollo XC26 mountain bike, convert it to a coaster brake fixie?
You can give it coaster brakes (but note that it won't be a fixie; it will be a single speed which can coast without peddling  with coaster brakes). Just buy or build a wheel with a coaster brake in it (you will need a coaster brake hub of the appropriate width) of the appropriate wheel size, put it in the dropout appropriately and then make sure to clamp on the reaction arm to the chainstay. 

That being said, coaster brakes for mountain biking are generally not a good idea. They don't dissipate heat very well, and you have less freedom to position your pedals. On top of that, coaster brakes often lock up which isn't great. On top of that, removing a coaster brake wheel is more work than just a quick release (for flats, for example).