...If you put Stuffit Expander 4.0/4.5 on your SE/30, then you will be able to open .sit files. You won't be able to open sit file made with later versions, but most software for that era Mac is available in the older .sit format anyway.

No HFVExplorer needed.

Please note: There appears to be a new application out called (Catacombae) HFS Explorer. It is apparently able to access .dmg and OSX .zip files. Never tried it. Needs Java Runtime.