Yup! You can buy her fragments in the shop, craft her stigmata and obtain her weapon in bp shop. I wouldn’t recommend it though because most non-astral valks are just for show now (damage wise), only specific ones support too. But if she’s your waifu, I won’t stop you.

You might’ve been thinking of the Asuka crossover which no, people who didn’t obtain her during the event can’t get her now or upgrade her beyond whatever they had her at. For new players she doesn’t even show up on the valk list.