McCallum, 25, was born in Glasgow, Scotland, but moved down under at a young age, has 68 caps for the Matildas and had a spell with Sky Blue in the Women's Professional Soccer League in the USA.
Alleway has just four caps and the 22-year-old is a rising star in Australia, having started her career with Melbourne Victory before moving to Brisbane.
Lady Imps manager, Glen Harris believes that the new additions show a sign of intent to the rest of the teams in the league.
"Laura is a young defender who we believe can do a job for us at the back. Collette is a midfielder from who has a vast world of experience.
"Her time in the American leagues has shown that she can win championships. This is something that we want to do in the WSL, and we are pleased to have her here.
"These are two great players, and I think that everyone else in the league will have to take notice now.
"This is a fantastic sign of our intentions, especially considering that we faced opposition from other clubs for the girls' signatures."
The signings represent Lincoln's first foray into the transfer market, having already lost defender Leanrda Little and winger Sue Smith to rivals Doncaster Rovers Belles.