If you wish to compile in custom patches to the version of qmail that plesk provides, there is an easy method. Simply download the sources for both qmail and the patches you wish to incorporate. After that, apply the Plesk patches which can be obtained here: http://kb.swsoft.com/attachments/1088/qmail_patches.tgz, and then apply your own. Provided that your patches do not modify the Plesk patches, you can then simply install the new binary.