Suppose you need to add a coupling in the middle of an existing length of pipe, such as the one shown below. Click on the pipe and select Subdivide Pipe.
Subdivide the pipe again a second time:
Delete the overall dimension. (This is a necessary step.)
Select and delete the pipe between the two subdivision points:
Add a coupling to one of the open ends:
Select the coupling and try to move it across the path of the other pipe. That means up or down in this case. Hit Yes when asked if you want to connect.
Here is the result:
You can also use this method to insert a tee or a pair of flanges etc.