I'd like to ask a question related to Nikon's SB-900 speedlight. However suggested tags are only 'SB-600' and 'SB-700'. I'm not able to add a new tags as my reputation is less than 300.
You can't until you get the rep. Just ask your question, someone will tag it appropriately if it makes sense to do so. If it's not really specific to the SB-900/910, then it probably shouldn't be tagged that way, so let's see first.
Having said that, I'm not entirely sure there's value in individual speedlight version tags. The SB-900/910 being a case in point, bit of a tag quagmire.