You're asking if it's ok to proxy Wraithguard as Grotesques?
If so, and you're just playing with people you know, as long as you OK it with them you're golden. For a tourney you'd likely need to do some conversion work to get them to pass muster, but for friendly games as long as they don't stick out like a sore thumb nobody should say anything.
But adding a couple spikes never hurt any eldar anyway.
I dedicate these deaths to Odin Allfather, Spearshaker, One Eye.
Fleet Shadowmaker - Kabal of the Dying Sun; Cult of Marrow Excised; Coven of Lambent Hunger
Sons of the Last Breath - Chaos Space Marines
Host of Shattered Purity - Chaos Daemons