In Logic Pro X you can merge patch settings from another patch with the current patch. This has several uses: for example, you can merge effects with a patch while keeping the existing instrument, or conversely try out different instruments while preserving the existing effects setup.
The patch merging area contains buttons for four types of patch settings: MIDI effects, instruments, audio effects, and sends. By default, all four types are selected, in which case selecting a patch replaces the current patch. By deselecting some items, you can choose which patch settings are merged with the current patch. When you choose a different patch from the Library, only those settings corresponding to the selected buttons are changed, while other settings remain unaltered.
If you deselect the Sends button, the channel strip (or strips) in the patch can access the existing project auxes (or any auxes created by existing tracks/channel strips).
This video shows one way to use patch merging in Logic Pro X. Here I show you how to use an audio bass patch on a virtual instrument.