tools:
something tapered that will fit in the pipe
a lighter
sealant
Method:
Heat up the pipe with the lighter, then jam in the tapered object and "stretch" the plastic pipe a bit - keep doin it till its wide enough to go on your pump.
Then seal it up with your sealant.
Job done!
OR
Get a strong craft knife and chisle away the plastic on the pump untill you have removed enough to get the pipe on - and again - seal it up.
I bodge everything - so thats my advice!
If you wanna do it properly, youll have to find some kind of coupler - im assuming both pipe and pump are of the same diameter?
If they are, you just want a short section of tube slightly larger in internal diameter than the outside diameter of your pump oulet/tube
and again - silicone it up!
MAKE SURE YOUR SILICONE HAS CURED BEFORE RUNNING THE PUMP!