the standard process for extensions requires a so-called Gap Analysis (https://www.fixtrading.org/fix-processes-and-templates/) to be submitted to the FIX Global Technical Committee (GTC) by a FIX working group or committee. This requires FIX membership which gives you other benefits as well. Looking at your specific request, it is a minimal extension of UnitOfMeasure(996). This field is a good candidate for a code list (see https://www.fixtrading.org/standards/codelists/) where we seek to simplify the process of an extension, i.e. adding valid values. UnitOfMeasure(996) may be converted to such a field in the future.
For now, please take “mi” as an abbreviation for miles. FIX will reserve this value as well as “km” for kilometers. Please note that this does not mean it is already part of the standard, e.g. it will not be visible in FIXimate. Let me know if you are interested in FIX membership going forward. As an exchange you can directly submit extension proposals to the GTC. We welcome more exchanges to become a part of the global standardization effort.