Question about plumbing?

My waste drain line is clogged and needs to be cleaned out. I have 3 plugs I can remove to enter the system and clean it, but none are too convenient. My plan is to add a clean out about 3 feed from where the line leaves the house, I believe the blockage is tree roots in this area. Here is the problem: I am not too familiar with the different plumbing fittings. The pipe exiting the crawl space underneath the foundation of the house is 3 inch ABS. I dug a hole 3 feet from outside the foundation and found the waste line was now steel or cast iron. It measures 3 3/8 inches o.d. as best as I can determine. I would like to cut this line and add what I think is called a two way clean out. I found one made of abs at a local hardware store, it is a slip to slip fitting. Is it ok to bury abs? What is the fitting called I would need to connect the abs clean out to the steel line? I think the steel pipe is called no hub. Do you have a better plan? Thanks