Udinese's Mohamadou Sissoko has started a second loan spell with Kilmarnock, who have also secured two midfielders.
Northern Ireland Under-18 international Jude Winchester had impressed on trial and comes from Linfield's youth team.
Jonathan Stynes arrives from Australian Football West Premier League outfit Joondalup.
Kilmarnock moved for 23-year-old Sissoko after losing central defender Ryan O'Leary to long-term injury.
Sissoko was one of Killie's main targets and Tim Clancy was released to Motherwell in order to free up wages.
He will remain at Rugby Park for the rest of the season having had a similar spell at Killie last term.
The signing of Sissoko may pave the way for fellow centre-back Patrick Ada to leave Rugby Park in this window.
Cameroonian Ada agreed a two-year contract in the summer after being released by Crewe Alexandra.
However, a clause in the deal allowed for it to be terminated early if the arrangement did not work out and the 26-year-old does not appear to fit into the style of play preferred by manager Kenny Shiels.
Killie have also been looking at Belgian international Mark De Man, who is a free agent.
However, a deal for De Man could be done after the transfer window closes on Wednesday night.
Meanwhile, Winchester is being seen as a long-term prospect and will join the Ayrshire club's youth set-up.