Yes, 1A in plural becomes 2A, 1B in plural becomes 2B.
So yes, but you may want to put the adjective first, as shown in the Bs, to emphasize the description aspect.

All phrases are OK.

• 1A, 2A are the most common and are equivalent:

  dans une chose pareille.

  dans des choses pareilles.

• 2B also exists, but is more stylish:

  dans de pareilles choses.

• 1B is correct but rare:

  dans une pareille chose.