That depends a bit on what you mean
ATK can handle a finite bias across the transport direction, this is the special feature of the software, and this of course constitutes an electric field.
General electrostatic fields in arbitrary directions are however much trickier to handle, as they must be incorporated in the boundary conditions in a proper way. Not only do they break the periodicity, which is used in the XY directions to simplify the calculations in ATK, but actually a constant electric field becomes an infinite potential if it extends to infinite, so unless you have something to screen the field (or you terminate it at some point, like a gate electrode), such calculations generally break down.
We are currently working on a new multi-gate model for ATK. It will soon be released in the upcoming new product from QuantumWise, ATK-SemiEmpirical which is based on the extended Huckel model instead of DFT, and will allow for an arbitrary collection of metallic or dielectric gates to be included in a two-probe system. The gates and their influence on the transport properties will be described fully self-consistently electrostatically, thus enabling a realistic description of transistor-like structures (and other devices).
This model willl then be included in the DFT code too later, by winter or so.
