I mean...theoretically maybe, but both Eternum and OiaL are structured very differently. OiaL hat basically two main LIs (Judie and Lauren) and various secondary LIs, some of which contributed almost nothing to the actual plot. In Eternum all the girls are main LIs with the only special case being Calypso. But all the girls know each other and all had and have their roles during vital plot points and story parts.
Eva was originally just a reminder to the player, that Eternum indeed is a porn VN.
And the point still stands, that it will be reasonably difficult to write the harem as is in a believable way, never mind adding another girl to the bunch that the others don't even know. It would make look Orion even worse. Even if the main LIs are all okay with being part of a harem, him having yet ANOTHER girl on the side? Nah...