I want to make my 1/2 bath in the basement into a full bath. The toilet already exists however it is to close to the cement wall to add a 2x4 wall behind it. So, that is going to have to be moved. The sink drains above ground. Is it easy to tie the shower into the toilet line that already exists.

You have me a little confused. What do you mean, " the sink drains above ground. ". Does it drain into a vertical pipe that vents out the roof? If so where does the sink drain connect?: I ask this because if you connect to the toilet line the shower will have to have a separate vent of its own but if you can connect the shower to the vented sink line it will be wet vented.
Regards, Tom

The sink I will be able to run into where the laundry tub drains into. I figure the less cement I have to bust up the better. I could run a vent from the shower to that part of the plumbing. It does vent back into the stack already. Since my stack is cast iron. I am going to assume that the plumbing for the toilet is also going to be cast iron. When I tie into it. I can use PVC right?

You tie into the main with a cast iron wye or combination wye and eighth bend and convert to PVC off the branch using a No-Hub band. If the sink drain is close to the shower you may tie into the sink drain and let the shower be wet vented by it. Regards, Tom

The toilet in the basement is already there and plumbed. Is there a wye already in place? I was thinking I could cut into the line coming from the toilet to the main and reconnect the toilet and shower at the new distances I need them.

The toilet in the basement is already there and plumbed. Is there a wye already in place? I was thinking I could cut into the line coming from the toilet to the main and reconnect the toilet and shower at the new distances I need them.

I keep attempting to tie the shower into a line that will wet vent it. This will save you from having to run a separate vent for the shower. You keep wanting to tie to the toilet drain forcing you to run a separate vent. Is there a reason for this? What about the laundry tub drain? If it's vented could you tie the shower to that line?
I don't understand the question," Is there a wye already in place?"
If you're asking if there might be a fitting already cut in that you could tie to I don't think so. If you tie the shower to the toilet line toy will have to roll a fitting up on a 45 degree angle downstream from the shower trap and take the vent off from that. Regards, Tom