In sys.txt I have a line that redirects to my sdcard. See below the second line in [folders].

[folders]
data_folder_name="iGO_Navigation"
content=/storage/sdcard1/iGo/content
linked_root_list="/extdata /sdcard2 /sdcard /sdcard/sd /sdcard/external_sd /mnt/sdcard /mnt/sd /mnt /storage /mnt/sdcard/sd /mnt/sdcard/external_sd /mnt/extsdcard /mnt/sdcard/extStorages/SdCard /ext_card /mnt/ext_card /mnt/extSdCard /mnt/ext_sd /mnt/sdcard/extStorages/SdCard /ext_card /mnt/ext_card /mnt/extSdCard /mnt/ext_sd /storage/extSdCard /mnt/storage/external_sd /storage/sdcard /storage/sdcard1 /storage/sdcard0"
;has_secondary_root=1
;android_secondary_root_path="/storage/sdcard0/addons"


In your case it should be:
content=storage/external_SD/addons/content/
When you add this line to your sys txt it should work. Take care you have the necessary items in the folder content on your sdcard.