Installing into an existing(!) partition

I have a problem: I downloaded the ISO of QNXRTP and tried to install it
into an EXISTING partition.

To explain why this is of utmost importance, i must explain a bit more:
I use the bootloader BootIt, which uses the concept of an extended MBR:
For every OS that can be booted, 4 partition entries for the MBR can be
choosen from a list of 255 or so (the EMBR, stored in an further
invisible partition). Especially with large HDs of today this can be
very nice. These 4 entries are copied at boot time from the EMBR to the

The drawback is, that if there are more than 4 partitions on the disk
(not visible at the same time, but nonetheless existent), the OS can’t
determine which space on the HD is free, because it sees only at most 4
partitions (one of which can be an extended one, of course, so there can
be more logical ones). That’s why i must not allow any other program
than the bootmanager itself create partitions, because it would almost
certainly accidently overwrite used disk space.

So, is it possible to install QNXRTP into an existing partition? The
automatic install just wants to create the partition on its own, which i
can not allow because of the reasons stated above.

Thanks in advance…

Clemens Dumat