It's not possible from NIP-17 to MLS - you should just start a new group with those same people. NIP-17 doesn't support groups at all though...
NIP-17 DMs work great for smol groups or 1on1 conversations. One question for the knowledgeable folks, is it possible to "upgrade" a group from NIP-17 to MLS once it reaches a certain size?
cc nostr:nprofile1qqspwwwexlwgcrrnwz4zwkze8rq3ncjug8mvgsd96dxx6wzs8ccndmcpzamhxue69uhkv6tvw3jhytnwdaehgu3wwa5kuegpp4mhxue69uhkummn9ekx7mqpzpmhxue69uhkummnw3ezuamfdejsul2dhz nostr:nprofile1qqsy2ga7trfetvd3j65m3jptqw9k39wtq2mg85xz2w542p5dhg06e5qpz4mhxue69uhhyetvv9ujuerpd46hxtnfduhszxrhwden5te0dehhxarj9enx6apwwa5h5tnzd9az7qgwwaehxw309ahx7uewd3hkctczcs834
Discussion
it does support multiple pubkey rooms https://github.com/nostr-protocol/nips/blob/master/17.md#chat-rooms