Wow MDK5 5.11a what a mistake.
I just tried working with a couple of different projects from MDK4, just to see how things go. One built and compiled fine in 5.10 and hopelessly fails in 5.11a. The other was never going to work in both. I tried working through some of the errors and windows to try to convert the projects.... waste of my time. Sticking with mdk4 until Keil figures things out. I don't have time to be a unpaid guinea pig for ARM.
And what is actually your problem? All MDK4 projects should compile flawlessly.
Did you install legacy support (www2.keil.com/.../legacy) software pack ?
Maybe you could an example of one of those errors?
I've converted a number of small and relatively large projects and found it quite straightforward.
Do you have much understanding of the toolset?