Are you referring to the connecting rod, the long bit that connects to the crankshaft one end and the piston the other. Or are you referring to the Gudgeon pin, which is the pin that attaches the piston to the connecting rod?Is it possible to attach crank pins (well, now I am lost with the language, anyway the part what is between piston head and crankshaft) without splitting the engine? So from bottom somehow? All advices on that would be welcome.

By the sound of it you want to slide the pistons into the cylinder up through the sump. I'm not sure if the piston will get past the crankshaft. I would have thought that would be in the way. You could change the big end bearings from the bottom. Why don't you want to remover the head (top end) off. Surly if the engine is out of the bike this would be simple and it gives you the chance to check the valves and things.
(:-})