well itachi said "simply gaining another's eyes didnt equate to gaining this power"
so id say sasuke wld hv to get the MS on his own first before he can get EMS
but i don think u can upgrade the EMS by taking more eyes coz if it was so, madara wld hv just taken itachi's eyes when he first met him along with the eyes of all the other uchihas who were capable of using the sharingan
look at the last panel here
uchihas killed their friends to get MS and took their sibling's
eyes to get EMS. I know itachi said earlier that the transplant cld only be done between clansmen but just think about it. If madara cld hv gotten the EMS from just anyone y wld he hv taken it frm his brother?