Another option is LSHIFT(LALT(Key_Minus)) right in the keymap, or #define KEY_SAM LSHIFT(LALT(Key_Minus)), and use KEY_SAM in the keymap.
This is a slightly underdocumented feature, but you can have keys on the keymap that have LSHIFT, LALT, RALT, LCTRL, LGUI, or any combination of these applied to their normal code. Mind you, this can only be done for keys defined in key_defs_keyboard.h.