Learn Dutch: Niemand steekt hier over.

Yes! Because the subject niemand (nobody) is indefinite (it does not point to one specific known person), you can use er to introduce the sentence. You will often hear Er steekt hier niemand over. Both mean "Nobody crosses here," but using er as a dummy subject is extremely common in Dutch and often sounds a bit more natural.

If I add er to the start of the sentence, what happens to the word order?

If you say Er steekt hier niemand over, the word er takes the first position. Because the conjugated verb (steekt) must always stay in the second position, the real subject (niemand) has to move further back in the sentence.

Why is over at the very end of the sentence?

It comes from the separable verb oversteken (to cross). In a normal main clause, a separable verb splits: the main part (steekt) takes the second position, and the prefix (over) gets kicked to the very end of the sentence.

Does niemand count as singular or plural?

Niemand is always singular, just like "nobody" in English. That is why we use the singular verb form steekt (stem + t) instead of the plural steken.