I looked on the web site, and couldn't find anything about applying for a corporate pilot slot. I do know that they go through the same exact interview as line pilots (I have seen them in the group). And yes, it is true that after 3 years, they can transfer to the mainline. They have to start over in pilot seniority. There has been some controversy about whether or not they keep their company (Fedex) seniority- I am not sure if they do anymore. The same is true if you get hired as a Professional FLight Instructor- you can come to the line after 3 years (in most cases.)
Sorry I only know my personal observations- and am not able to give you a clear answer.