Why to use a built-up steel cross-section?
IDEA StatiCa Connection can add fixed support only to one member. If you have a connection like the one in the figure, you need to use a composed cross-section (for the column).
You could use several predefined types from the Cross-section navigator
or define your own type in the General cross-section editor.
You can design joints defined by general cross-section with several parts to satisfy your needs.