Can I add cocoa butter to white chocolate chips to promote a smoother melt?
White chocolate melts smoothly if there's nothing wrong with it. While I'd sometimes melt milk or dark chocolate in a microwave, for white chocolate I always use a bain marie, heated gently and stirred very gently.

If you try to heat it too fast it will catch and never melt. This can also happen if it's old, or has been stored in fluctuating temperatures.

There's a lot of cocoa butter in white chocolate anyway so adding more wouldn't do much to the melting. It might however be desirable to make the chocolate less sweet.