I've heard that by using Wrye Bash you can exceed the 255 mod limit that Skyrim has, but also merge certain mods together to lower compatibility issues.

I've just installed it and my question is, can I "bash" all the mods I've already installed via NMM, or do I have to uninstall them ALL and start again?

I've tried googling "Using Wrye Bash with NMM" but all it comes up with is tutorials for starting fresh

EDIT: Also, the OBVIOUS question I've left out is, HOW would I do this?

Depends on what your using BASH for.

To get the full functionality of Wrye Bash, which can install and remove your mods for you, it has to install them or at least monitor the installs. However, if your using BASH to make a Bashed Patch or to Merge some mods together, then you can do that after NMM has installed the mods. Thing is, it is best to use one Mod Manager to instal the mods, no matter what it is, this way you have control over your mod install and uninstalls in one place.

I don't use BASH to install mods, just for getting them to work together. Note, I have only merged patches, never mods using BASH. Obviously a Merged mod would not be manageable by NMM or other mod managers. I also found that Merging Mods makes troubleshooting issues that much more difficult. And, I would never merge any mods where one or more of them are still being developed or updated.