This will certainly seem simple to you but I have a part with custom properties and my client has his own parts without any custom properties and we would like to extend my custom properties to all his parts. I admit that I don't know how to do it and that there is surely an easy solution
If you have created a custom properties file, you can set its properties to all parts by selecting all parts in the head assembly including the prieces in the subassemblies and then clicking on the icon to the right of the custom properties. The same thing happens when selecting all the assemblies.
There is also the batchproperties utility in the mycad tools which is done to retrieve the properties of a part and apply them to a directory or multiple directories in a few seconds. (with batchproperties it modifies the properties without opening the file so extremely fast.
Yes, the utilities of the myCADtools suite are there for that. If you have not subscribed to the myCADservices subscription, you can still download the myCADtools setup in order to test the utilities that may interest you (1-month trial period) and thus be able to see how useful and simplified your daily life is.