If I understand you correctly, you’re asking if it’s possible to feed an analog SMPTE timecode signal into an audio input and have Cubase sync to that? Short answer: no. There are interfaces which can do this (e.g. some MOTU) but they then send MIDI timecode (“MTC”) to Cubase, which is what Cubase needs to see in order to sync to tape.
However I think you might need to step back a bit an consider what you’re trying to achieve. There are two different areas of synchronization here, audio clock sync (which you achieve using a suitable audio interface and making the RD8 the master) and machine control sync, which you could most easily do using e.g. a device that could convert the 9-pin ADAT sync control to MTC. There are a number of MOTU devices that can do both, but it would be non-trivial to set up and get working flawlessly.
Stepping even further back, I’m wondering what you’re trying to achieve here in the first place? ADAT tape is hopelessly outdated and any decent audio interface is going to give you far superior results … and let’s face it, who wants to wait any more while tape is being spooled?
On the other hand, if all you want to do is transfer multitracks from ADAT to disk, you don’t need SMPTE sync – you’ll need an audio interface with an ADAT input and then simply set your audio interface (not Cubase!) to slave to the ADAT input’s audio clock; in Cubase, route each ADAT channel to a separate track, hit record and play the ADAT.