Q: I have a custom controlled vocabulary set that I use for keywording in Photo Mechanic. Is there any easy way to use it in Lightroom?
A: Those are two of the easier formats to share as both understand and can use both synonyms and excluded category headers.
Some differences between the two are addressed in the CVKC-FAQ page.
For members of the Controlled Vocabulary Forum there is a document in the Files section about editing a Photo Mechanic style Keyword Catalog using a plain-text editor. This explains a bit of the underpinings of the format if you want to amend your version in a text editor before importing into Lightroom.
Depending on whether you are on Windows or Mac, it may be possible to simply save out the file from Photo Mechanic and then import into Lightroom. The Controlled Vocabulary Keyword Catalog Support pages for both applications have basic
directions on how to export / import a Keyword Catalogs such as the Controlled Vocabulary Keyword Catalog.
The only issue might be that Lightroom is expecting a Unicode "UTF-8" file with Mac carriage returns, and Photo Mechanic may save out the file as Unicode "UTF-8" but with Unix Line Feeds.
If you need to handle those types of conversions, utilities such as TextWrangler, TextPad, or Jedit can be very useful.