But can we boost that signals - focusing not on observation, but on detection of objects in the solar system moving toward Earth? With such great resolution, what power, scanning frequency etc. of an active radar would be needed? Considering the area we can't use focused beams (but impulses would be nice) - right?, could the solar radio emission be used instead or the light is better? Or maybe there is some background radiation on frequency that we can detect when is covered ?

Wouldn’t the area of the combined sensor be the sum of the areas of each sensor?

Yes, if that’s what you’re asking (which is not what I thought this was about). Thought as someone else said, even combined it’s still less than Arecibo.

> it’s still less than Arecibo

Which is a huge baseline and getting anywhere near it with so much better resolution could be really impressive.

We’ve basically already got one twice the size of Arecibo, though: https://en.wikipedia.org/wiki/Square_Kilometre_Array

Also cool. But the same applies: If an orbital system could get anywhere near SKA with so much better resolution it could be really impressive.

If it's a third to half the dish area of Arecibo, and Arecibo is half of SKA, then it qualifies as "anywhere near". And SKA goes up to 14GHz so this theoretical system is completely incomparable on resolution.