I write a lot of custom modules. I normally end up needing to add Features to my existing modules well after my custom module has been established. I used to manually merge my existing module with the generated Feature by renaming files and copying and pasting the generated code/files.
Make your existing module a Feature by adding the following lines to your module's info
file.
package = "YourFeatureGroupName" project = your_module_name version = 7.x-1.0 features[features_api][] = api:2 project path = sites/all/modules/custom
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