On each row place two letters that can be attached to the end of the word to the left to give a longer word.

Very easy!!
On each row place two letters that can be attached to the end of the word to the left to give a longer word.

When completed, the eight added letters will give a word reading downwards. What is it?

MET _ _
CONS _ _
SOCK _ _
HAS _ _


METRO
CONSUL
SOCKET
HASTE
Answer: RO+UL+ET+TE = ROULETTE