I am finding it very difficult to get a roll away bed put in a double room (or even a suite) without having to pay the price of another room. Would like to stay at the Taiwan Hotel in Beijing but no luck so far with doing it online or through the 800 numbers. Any advice on different hotels that would be near central/shopping area and for under $100 (Nov 2 - 5).

I hope a Beijing expert will come in and give you some tips, however, meanwhile, getting an extra bed I can't see should be any problem and certainly no need to get 2 rooms, that applies almost anywhere is Asia.
I don't know Beijing but my wife's brother does a lot of travel on business and he gets me to sort it for him.
Beijing New World Apartment Hotel next to central railway station, gym, swimming pool, sauna, bar and restaurant, I looked for November and 1 bedroom apartment with living room for 3 people $99, he went once and went again so must be ok.
Days Inn Joiest in the city offers king room with extra bed and inc breakfast for $83 for 3 people. They also offered before the Qian Yuan International Business Hotel about $80-85 for triple.

I got those off Nongasia, www.nongasia.com/hotels.html, just go to Beijing and click, they use different services depending on destination, for China normaly Hotelclub ( Australian and efficient ) but works fine and 'most' are instant confirmantion, they have 150 plus places in Beijing.

Others I am sure will add their tips but as far as not able to get a 3rd bed without getting another room, and a budget of $100 it would be 'no problem at all'

I had the same trouble trying to find a room for 3 people and was told by all we needed to book 2 rooms. I finally called one of the hotels we will be staying at and they said when it says "2 twin beds" the beds are double or full size and not 'twin' as we have in US. Not sure if this is for all hotels that say '2 twin beds' or just the Peninsula.

you just ask them add one bed in the room,which will cost 100 yuan more.for you budget,i suggest Park Plaza,which is a new hotel,5-star hotel,4-star hotel`s price.about 100USD a night.

2nd Violet's recom of the Park Plaza, great location steps to Wangfujing, new and comfy, and eager-to-please staff, far better than the Taiwan which is getting a bit worn.