These systems are not too hard to do. It sounds like you already have a baset in the basement and a pump. My suggestion would be to interrupt the line going outside, and divert it to the interior plumbing line for the upstairs fixtures. You should tie into the horizontal or vertical line with a wye and 1/8th bend fitting. If your existing plumbing is plastic or cast iron, you can cut it (with the proper blades) and use stainless steel banded rubber transition couplings. These allow you to splice a wye in the middle of your piping. The best thing is to tie into the sewer line on a 3 or 4 inch pipe with a 2" branch for the sump. Put your wye together with 2 short nipples in the larger hubs, then measure the overall length and cut out a section of sewer line downstream of the vent for the toilet or any other fixture. Do not cut the line between the toilet and the main stack. Make sure you cut the section 1/2 inch larger than the piece you want to install. This gives you some room for the rubber stops in the coupling. But make sure you support the line with strapping so it doesn't fall when you cut out your section.
Then install your new tee and clamp on your rubber couplings. When this is done attach the line from the sump, making sure to grade the pipe towards the new tee.