Killie have endured a difficult campaign so far and sit third bottom of the Scottish Premiership.
The defence has been a particular concern for Johnston - they have conceded 28 goals so far - and he already has a centre-half on trial ahead of the transfer window opening.
Johnston said: "You are always looking. We had someone in on trial today and we will continue to do that throughout January and assess the squad then.
"We've got a foreign boy in just now. He has been in for a couple of days and
we had someone in last week as well.
"There will be a few people coming and going over the next month because you've got to make sure you bring the right people in.
"We are looking to strengthen a few positions. We are in the position we're in in the league for a reason and we've got to improve.
"We've definitely got good players here and if we start putting a run of results together we can quickly work our way up the table."