Battlescribe has its own way of doing things, I wouldn't put too much faith in the restrictions it imposes on you. Far as the unit entry, Arena Champion can take an agoniser, a heat lance and a grav talon with no problem even if the Battlescribe does not. What I do is just do it the way Battlescribe makes it, then tell the opponent that the champ has the extra gear - works out fine with our player group.
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Siticus Empyrean Vision - my Facebook page with various painting projects
Siticus' Empyrean Visions log - the project log for my Aeldari works and beyond