If I add a new feature (called “feature X”) to existing software, what do I say about the action?
I added support for feature X
I added support of feature X
Or something different?
Neither one. You’re not adding support of the feature; you’re adding the feature itself.
First, let’s address your premise:
If I add a new feature (called “feature X”) to an existing software…
should be either:
- If I add a new feature (called “feature X”) to existing software…
- If I add a new feature (called “feature X”) to an existing software application…
(Either eliminate the “an“, or change “software” to its adjective form. “An existing software” sounds off.)
Now, to answer your question, simply say:
- I added feature X to the software.
- I added feature X to the software application.
As for when to use the word support: if feature X had already been added, and support for the feature was added some months later, you might say:
- I added support for feature X in the software.
(I’m assuming that feature refers to some new capability, and support refers to some support for that capability, such as built-in help menus.)