Bought a triplex in 2011 with all cash in my retirement account.

Just ensure the agreement is clear and formalized.Pooling reserves across properties might make sense if you’re confident in your cash flow and other contingencies.

With the money from my savings and stocks I have about 100k . 

my condolences on your recent lost.there are a few ways you can do this, as mentioned above you can take a heloc on your primary residence which will give you a second mortgage, first mortgage payment wont be impacted. you can do cash out refinance which will give you one mortgage and payment. it will pay the old mortgage and give you money in your pocket to pay for the rehab.or you can do a renovation refinance which is similar to cash out refinance but we would contractor involved  as part of the refinancing and they are paid through the loan

It may make sense to save a bit more and invest from a position of power.Keep in mind most helocs are adjustable.

Consider using DSCR loans- Cash flow of the property is considered in lieu of your income3.Consider purchasing in a different asset class (Large multifamily) where borrowing will be a function o fthe property income, not yours

In my analysis the cost for an ADU in kern county would run about $85K and I have saved about 75% of that right now.  

Most of us will go even higher...say 85%...but you have to get the property to cash flow at that level.

There are a lot of options out there.Some of the most useful items BiggerPockets members have identified are features that save time so they can spend time working on their next property.

If you have to continue saving do that.