Convert track bike to geared
It's not ideal but is doable.
Biggest factor being the rear hub spacing.
If it's 120mm horisontal dropouts (the standard for track frames) then the best thing would be to get a 120mm internal geared hub such as the Sturmey Archer S-RF5 5Spd Hub (other available).
If, for any reason, it has 130/135mm rear dropouts (as found on jump bike and other single speed mtb frames), then you can use a horisontal dropout mech hanger, pic below:

I would really think twice before bending a 120mm dropout to accept 130mm wheel, depending on the frame age and material, this could be a very bad idea.
Another thing to consider is the gear cable, best thing would be use a complete outer running from the shifter to the rear and secure it along the way.