Take another look at the original sentence:
You wanted to fix it this way:
Adding because, a subordinate conjunction, in front of her long fingernails made accurate typing impossible creates a subordinate clause. You can, in fact, fix a comma splice by subordinating one of the main clauses. However, when a subordinate clause follows a main clause, you generally do not use a comma. The comma in front of because makes this choice wrong.
You might want to consult the rules for fixing comma splices and fused sentences.