Hi. I need to instal a new toilet to an existing vertical and rigidly fixed, internal soil pipe. Breaking into the pipe will be easy, but, as I can't move the soil pipe up or down, how do I reconnect? Are there such thing as adjustable straight fittings?

You will need to excavate under and around the drain pipe. Do not break into the pipe, cut it with a wrachet cutter or reciprocating saw so you can install a PVC 4" wye. Arrange the wye to go downsteam with the old pipe.

You will need to excavate under and around the drain pipe. Do not break into the pipe, cut it with a wrachet cuuter or reciprocating saw so you can install a PVC 4" wye. Arrange the wye to go downsteam with the old pipe.

Thanks for the advice, ballengerb1, but... The soil pipe runs vertically through the ceiling from the apartment above and through the floor to the apartment below. There are toiets running into the soil pipe both above and below. The pipe, therefore, cannot be moved either up or down to fit anything. Any thoughts or suggestions?

It sounds like you're asking for a way to connect the toilet to the vertical stack. Am I correct? There are several ways to do this. The easiest way would be to locate a rear discharge toilet and connect directly to the stack. This toilet will set on the floor.
Another way would be to build a wall for a wall hung rear discharge toilet. This would involve installing a chair carrier in the wall to mount the toilet on.
Your choice. Good luck, Tom