Can Photoshop match merge a layer to an existing layer in same file ?

Hi,

CS6

If I am making a printscreen of a large image bit by bit and pasting as I go into a .psd file, I normally make the addition 50% transparency and move it around, nudging with arrow keys turning visibility on and off until I get it matching, then combine the layers (after making the 50% as 100% again) and then bring the next one in and repeat the process.

Very laborious.

Can I select two layers and say to pshop match together, photomerge in other words, within the psd file,

so printscreen, paste, select previous layer, merge,

repeat process.

Otherwise its bring all the printscreens in, then generate separate files, save them all, then photomerge.

I am unable to generate files from layers, Fules>scripts>export layers to files   ... I get an error.

or make a file with each paste of a printscreen then save them then photomerge.

It would be great if I could merge as I get two layers.

Merlin3