You are going to make a mess no matter what. Getting the slides in kinda depends on where the leak is and how bad it is. If the leak is on the extension side you should be able to use the hydraulic pump to bring the slides in. If it is on the retraction side and the hole is small, you might be able to use the pump again. Either case, you might need to top up the reservoir as you go.
You can push the slides in by hand but you need to relive the hydraulic pressure to do so. Either run the pump as you push or disconnect the hydraulic lines from the ram.
Once you have the slides in, you need to figure out how to keep them in. Normally they are held by hydraulic lock. Without hydraulic pressure to hold them, the slides will come out. Not something you want to see going down the road.