Corten is basically a low alloy steel. Low alloy steels are generally weldable to themselves or to stainless steel using stainless steel weld metal, particularly if the stainless side of the joint is one of the 300 series like Type 304L. Since the design stresses for the structure are not likely to be high, it should not be difficult to develop a welding procedure for your application. You can probably start by considering the use of the same austenitic stainless or nickel alloys commonly used to join steel to austenitic stainless (assuming the stainless that is eventually selected is one of the common austenitic grades).

To back up what Bill A said, corten has a little bit of copper added to arrest corrosion, but otherwise it can be treated like most other low alloy steels, which are welded to stainless steel on a regular basis. The metallurgy of the issues are well known, and any reputable fabrication shop should not find this to be a problem.

As the others have mentioned, joining corten (ASTM A588) to a 300 series stainless steel is no more difficult than joining mild steel to 300 series stainless. Typically, an E309 electrode is used and although a little more skill is needed for welding stainless out of position, this is not a difficult weld. Minimum tensile strength of the A588 is slightly lower than something like 304 (63-70 ksi vs. 75 ksi), but the weld can easily provide a matching tensile strength. The difference in thermal expansion coefficients and potential high temperature corrosion issues related to dissimilar welds would not be an issue for your architectural application either. If the majority of the structure is corten, there probably won't be an issue with galvanic corrosion, although you should have someone look at the specifics of your structure to be sure.