Although it’s not yet listed as a verb in the on-line Oxford or Merriam Webster, you are absolutely correct that native speakers can get away with using just about any word as a verb. Those dictionaries that I mentioned are necessarily abridged, since the English language has nearly a million words, and it’s just not practical to list all of them with all their definitions, especially when it comes to technical terminology. If it’s used in your field, then I see no problem with it. A web search shows that “threshold” is used frequently as a verb in certain scientific fields, as well as in the technical field of digital image processing. Here are some of the dozens of websites that use the word as a verb: http://zone.ni.com/reference/en-XX/help/372916L-01/nivisionconcepts/thresholding/ http://zone.ni.com/reference/en-XX/help/372916L-01/nivisionconcepts/thresholding/

No, threshold is a noun and it doesn't exist in any other form. Is there another type of expression you'd like to try to use?

No.

No.