extensible; expandable
Used in technical contexts (software, hardware, systems) to describe something that can be extended or expanded in functionality or capacity.
This software has an extensible design.
Build an expandable system.
拡張 is the noun meaning 'extension' or 'expansion'; 拡張可能 adds the sense of 'capable of being extended'.
可能 means 'possible'; when attached to 拡張 it forms a compound meaning 'extensible'.
Compound of 拡張 (extension) + 可能 (possible). A straightforward Sino-Japanese technical term.