I have an installed 3/4" inch tongue and groove floor in the majority of one floor of my house. I want to complete the floor by installing two more rooms with the same flooring material. I found the same flooring materials as what was installed. The people that installed the floor removed the tongues. I want to some how create grooves to tie in the new flooring perpendicular to what the previous guys did. Is there a way to route an existing installed floor? I was thinking a router, but what do I do when I bump into the wall and it can't complete the groove to the end?

Also in another area of the floor they extended the planks to completely cover the threshold of a door jamb. I think it would look odd if all my planks started on the even line, that is not even in the middle of the threshold. What I was thinking about doing is cutting back all the planks 5", routing a groove, install an plank perpendicular creating a break in teh pattern so the new floor would not have to exactly match the exiting planks lines of the other room. What kind of tool could I use to cut an even line in the exiting floor to split the threshold?

you might just as well pull that board up and put a new one with a grove or just use the router like you mentioned. You dont have to go all the way tro the end just dab some glue near the ends where the router has stopped and spline the middle. as for cutting a straight line into an exsisting threshold you best bet is a multimaster or similar saw. Use a straight edge for a guide, score the cut line with a razor knife to help guide the saw cut then just go slow and steady with the saw. You may want to practice on a scrap to get the hang of the saw.