This work explores a new method of simultaneous membrane formation and chemical modification in a spinning process for fabricating asymmetric composite hollow fiber membranes. This method is based on controlled crosslinking reaction between the membrane forming polyimide P84 and the chemically modifying poly(ethylene imine) (PEI) dissolved in the bore liquid. We study the interplay between phase inversion and crosslinking, which determines the final morphology and chemistry of the hollow fiber membrane. Depending on the composition of chemically active bore liquid, two different types of membranes can be fabricated. The first type is a membrane with a dense, crosslinked, gas selective inner layer (thickness in the range 1.4–6.1 μm) having a mixed gas CO2/N2 selectivity 12 in the presence of water vapor. The second type is an entirely crosslinked membrane which is not soluble in NMP; it is porous (pure water flux 44.4 ± 1.2 l/(m2 bar h)) and selective for the separation of similar size proteins, bovine serum albumin (BSA) and hemoglobin (Hb), due to the charge of the membrane. The isoelectric point of the new membrane is basic (pH 9.3), much higher than that of the P84 membrane (pH 3.75). The ease of process operation (only the bore composition needs to be modified) opens new perspectives towards applications in the area of gas separation and/or liquid filtration, the latter especially in harsh solvent environments.