A bottling bucket should generally be used for bottling. If you must rack to a secondary then you should get a 5 gallon bucket (assuming this is a 5 gallon batch) so you limit the amount of headspace.
I also question how airtight the bucket is through the spigot. If tightened wrong, the bucket can leak, I know this because I've done it. Also, you really need to sanitize the spigot and every single thread. It seems so much easier to buy a second fermentation bucket to actually use as a secondary.
As for adding priming sugar, you'll have to mix it which is a bad idea. If you rack to a bottling bucket when you siphon you will mix the priming sugar. So again, it seems best to get another bucket.
Why are you using a secondary? Just curious.