Learn Farsi: این شرکت موقتاً تعطیل است.

The word موقتاً (movaqqatan) is made of the adjective موقت (movaqqat, meaning "temporary") plus the suffix -اً (-an). In Farsi, this suffix is borrowed from Arabic and acts just like the English "-ly", turning an adjective into an adverb of manner.

How do you pronounce the ending of موقتاً and what is that symbol on top of the alef?

It is pronounced "-an", making the whole word "movaqqatan". The two small diagonal lines are an Arabic diacritic called "tanvin". When placed over an alef at the end of an Arabic root word, it creates this "-an" adverb sound.

Could I attach this -اً suffix to a native Persian word to make an adverb?

No, this suffix can only be attached to words borrowed from Arabic, like موقت. You cannot add it to purely Farsi adjectives (for example, you cannot add it to خوب to mean "well"). Native Persian adverbs are usually just the adjective form left unchanged, or formed with other Persian suffixes.

Can I use تعطیل to say that a door or window is closed?

No, تعطیل (ta'til) is specifically used when a business, school, or service is out of operation, closed for the day, or on holiday. To say a physical object like a door or window is shut, you would use the word بسته (basteh).