Our home is built on a concrete slab. We build it that way because of poor drainage on the lot. It is a two-story home.
We put in a small bathroom on the first floor with a sink and commode.
All of the plumbing is centralized. Now, we want to remodel the small bath downstairs and add a shower; however, we do not want to have to break up the concrete to put in a drain for it.

Is is practical and/or probable to tie in the drain for the walkin shower to an existing pipe which carries out the water from the sink and/or washer which is also downstairs.

What do you suggest?

The drain for a shower is underneath the shower. So you would have to elevate the shower from 6" to 11" depending on the distance from the vertical stack. I think it's easier to break the floor. Certainly the job is permanent when you rough in below. Many pedestals fail over time.
You can tie into the line that serves a kitchen sink and washing machine but you will need a vent. Three fixtures wet venting on a 2" line will probably not work. If these fixtures have two different vents then you can tie the shower in below the sink but not below the WM without a vent. You need to be within 5 foot of a vent.