We present what we believe is the first conjugate adaptive optics (AO) extension that can be retrofitted into a commercial microscope by being positioned between the camera port and the image sensor. The extension features a deformable phase plate (DPP), a refractive wavefront modulator, and indirect wavefront sensing to form a completely in-line architecture. This allows the axial position of the DPP to be optimized by maximizing an image quality metric, which is a cumbersome task with deformable mirrors as the correction element. We demonstrate the performance of the system on a Zeiss AxioVert 200M microscope equipped with a 20 × 0.75 NA air objective. To simulate sample-induced complex aberrations, transparent custom-made arbitrary phase plates were introduced between the sample and the objective. We demonstrate that the extension can provide high-quality full-field correction even for large aberrations, when the DPP is placed at the conjugate plane of the phase plates. We also demonstrate that both the DPP position and its surface profile can be optimized blindly, which can pave the way for plug-and-play conjugate-AO systems.