Can I merge a separate iTunes account with my phone?

Merging content from different Apple IDs into one isn't supported. See Authorization vs. association - Apple Community for general background. If you have media on a device that isn't in your library see Recover your iTunes library from your iPod or iOS device - Apple Community. Once it is all in one place you can add whatever content you want back to the device.

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See this thread for some suggestions on how to merge your data with minimal loss. Apple ID - Apple Community

App purchases and app subscriptions and such cannot be transferred.