That may or may not work as expected.
For Artificer Battlesmiths, for example, you can add a Steel Defender as an extra. However, some of the bits in the statblock depend upon the Proficiency Bonus of the character, and they seem to have implemented it in some way that you cannot customize.
To wit - if you take the Steel Defender creature you can add as a creature, customize it, and add it as your steel defender some elements of the stat block you see when you expand the listing in your Extras do not reflect customizations.
Basically, you can't get the saves and skills of your Steel Defender to display the right numbers at this time.