>>716438421
Yes, Eldar can technically travel in the Warp aboard an Imperial spaceship, but it is extremely rare, risky, and usually only happens under unusual or desperate circumstances. Here’s how it breaks down in Warhammer 40k lore:

Possible – but Uncommon
The Warp is dangerous to Eldar: Since the Fall of the Eldar, their souls are especially vulnerable to Slaanesh, who constantly hungers for them. The Webway was developed as an alternative to Warp travel to avoid that risk.

Traveling the Warp unprotected is a huge risk for Eldar — if they die in the Warp, their soul is almost certainly lost to Slaanesh unless captured in a spirit stone.

Imperial Gellar Fields (used to protect ships from Warp entities) would technically protect an Eldar passenger too. However, they are designed with human souls and psyches in mind, and may not be fully reliable for xenos.

Lore Examples of Eldar on Imperial Ships:
There are a few instances in Black Library novels and lore where Eldar (usually Craftworld or Corsair) individuals have cooperated with humans and may have boarded Imperial vessels.

Inquisitor Eisenhorn and Rogue Traders sometimes deal with Eldar or carry them under special conditions — though not always through Warp jumps.

Yvraine, the Emissary of Ynnead, interacts with Imperials, though she travels mostly via Webway. But it's an example of factions cautiously interacting.