I'm normally an advocate of the Silver Plan, but since you already have APs this plan is not for you. It can only be purchased with the Dream Maker package, and includes your resort stay and UPH passes.
But... here's the info you were looking for in case you haven't activated your APs yet, and want to save them for a future trip. The cost of adding on the Silver Plan is $68 per adult per night. Since you get two wishes per adult per night, each wish is worth approximately $34. As long as you will be using the wishes for things that cost more than $34 you'll do fine.
For instance, Cirque is a good choice (saves about $10 per adult), as are the tours (saves about $30 per adult) and the dinner shows (saves about $20 per adult). From what I've heard spa treatments come with a good savings as well. But, on the flip side, dinners (unless you share meals or eat a lot) are generally not a good way to save.
Keep in mind also, that you get one freebie with the Silver Plan (a choice of souvenier, minigolf, Leave a Legacy tile or $25 Planet Hollywood gift certificate). You also get preferred viewing locations for Wishes, Illuminations, and the AK parade. While I think the locations stink, others enjoy having a definite seat.
Oh, and to answer your questions about whether you could add the plan to only part of the trip... the answer is yes and no. You can't split a single reservation up and add the plan onto one part of it. But, you can split your stay into two separate reservations and add the plan to one of those reservations. We did this by booking a Dream Maker package with the Silver Plan for the frist three nights, and booking a separate regular Dream Maker package for the last three nights. Worked out great, but we did have to use up all our wishes during the first half of our stay. You could do this by booking the Dream Maker plus Silver for the first few days of your stay, and then booking an AP room only package. Could actually work very well for you.
Definitely something for you to think about.... good luck!