Normal fixed blade as sgian dubh? No, a normal fixed blade isn't a sgian dubh. Can you wear it. I mean, you can technically wear whatever you want so absolutely. It will likely look a little odd to wear something else rather compared with just not having it.

To give an example of another knife used in cultural dress, the kirpan. Cool knife, goes with the culture it's part of. Now imagine exchanging that for another knife whilst still wearing the cultural attire that's associated with it. A different knife would look a bit out of place. Not necessarily wrong person se but generally noticeable. It's a bit like that.

As a side note, I'm not really sure why you seem to think a sgian dubh has to be dull? A lot of them are sold that way because kilts are often worn at functions with alcohol and that doesn't always mix well with pointy things. It's in no way a requirement though.