diff --git a/ports/esp32/Makefile b/ports/esp32/Makefile
index c8ca9262c8..88f946fa98 100644
--- a/ports/esp32/Makefile
+++ b/ports/esp32/Makefile
@@ -14,7 +14,7 @@ BAUD ?= 460800
 
 PYTHON ?= python3
 
-GIT_SUBMODULES = lib/berkeley-db-1.xx
+GIT_SUBMODULES += lib/berkeley-db-1.xx
 
 .PHONY: all clean deploy erase submodules FORCE
 
diff --git a/ports/esp8266/Makefile b/ports/esp8266/Makefile
index 52ba18d7d8..c027f690fb 100644
--- a/ports/esp8266/Makefile
+++ b/ports/esp8266/Makefile
@@ -32,7 +32,7 @@ FROZEN_MANIFEST ?= boards/manifest.py
 include $(TOP)/py/py.mk
 include $(TOP)/extmod/extmod.mk
 
-GIT_SUBMODULES = lib/axtls lib/berkeley-db-1.xx
+GIT_SUBMODULES += lib/axtls lib/berkeley-db-1.xx
 
 FWBIN = $(BUILD)/firmware-combined.bin
 PORT ?= /dev/ttyACM0
diff --git a/ports/mimxrt/Makefile b/ports/mimxrt/Makefile
index 13b766eb3c..6312ecdd8f 100644
--- a/ports/mimxrt/Makefile
+++ b/ports/mimxrt/Makefile
@@ -7,7 +7,7 @@ BOARD_DIR ?= boards/$(BOARD)
 BUILD ?= build-$(BOARD)
 PORT ?= /dev/ttyACM0
 CROSS_COMPILE ?= arm-none-eabi-
-GIT_SUBMODULES = lib/tinyusb lib/nxp_driver lib/lwip lib/mbedtls
+GIT_SUBMODULES += lib/tinyusb lib/nxp_driver lib/lwip lib/mbedtls
 
 # MicroPython feature configurations
 FROZEN_MANIFEST ?= boards/manifest.py
diff --git a/ports/nrf/Makefile b/ports/nrf/Makefile
index 0586524ba6..23be4430f3 100644
--- a/ports/nrf/Makefile
+++ b/ports/nrf/Makefile
@@ -58,7 +58,7 @@ FROZEN_MANIFEST ?= modules/manifest.py
 include ../../py/py.mk
 include ../../extmod/extmod.mk
 
-GIT_SUBMODULES = lib/nrfx lib/tinyusb
+GIT_SUBMODULES += lib/nrfx lib/tinyusb
 
 MICROPY_VFS_FAT ?= 0
 
diff --git a/ports/samd/Makefile b/ports/samd/Makefile
index 7091365f49..aed8637abc 100644
--- a/ports/samd/Makefile
+++ b/ports/samd/Makefile
@@ -24,7 +24,7 @@ FROZEN_MANIFEST ?= boards/manifest.py
 include $(TOP)/py/py.mk
 include $(TOP)/extmod/extmod.mk
 
-GIT_SUBMODULES = lib/asf4 lib/tinyusb
+GIT_SUBMODULES += lib/asf4 lib/tinyusb
 
 INC += -I.
 INC += -I$(TOP)