Learn French: Des épices ? J'en ai ajouté dans la poêle. - Spices? I added some in the frying pan.

This is the main exception to the rule about agreeing the past participle with a preceding direct object. When the preceding direct object is the pronoun en (meaning "some" or "of it/them"), the past participle always remains invariable. So, even though épices is feminine plural, ajouté does not get an -es at the end.

Could I say Je les ai ajoutées dans la poêle instead?

You could, but it would change the meaning. Je les ai ajoutées means "I added them" (referring to specific spices, like "the ones you bought"). Because the first sentence asks about Des épices ("Some spices"), you must use the pronoun en to say "I added some". Notice that if you use les, the normal agreement rule applies and you do add the -es to the past participle.

Why does the sentence start with Des épices ? instead of just Épices ?

In English, we can drop the article and just ask "Spices?". French, however, requires an article before almost all nouns. Because you are talking about an undefined quantity of a plural noun, you must use the plural indefinite article des.

Is poêle masculine or feminine? I've seen it mean "stove" before.