* -able: This suffix suggests something can be done or is capable of being something. So, you could use "welcoming-able" to describe something that is able to be welcoming. This isn't common usage, but it's grammatically correct.
* -ness: This suffix is used to create nouns from adjectives. So, you could use "welcomingness" to describe the quality or state of being welcoming.
* -ly: This suffix is used to create adverbs from adjectives. You could use "welcomingly" to describe something that is done in a welcoming manner.
Ultimately, the best suffix to use will depend on what you're trying to express.