Can I use my unsweetened baking chocolate as semi sweet chocolate

It depends on what you are trying to make. For example, Ghirardelli recommends subbing 4oz of their semisweet chocolate with 2oz of unsweetened and 2 oz of sugar.
Also, here is a link to the discussion here on Food52 https://food52.com/hotline/1356-i-have-a-lot-of-unsweetened-baking-chocolate-what-s-the-correct-ratio-of-sugar-to-add-if-i-want-to-u

They're not the same and I would not recommend it. I generally think unsweetened chocolate should be reserved for instances where it is called for.