The addy software has the ability to enable investors to invest Canadian dollars into a US dollar issuance but it is up to each issuer to decide if they will allow it.
If the US offering that you are wanting to invest in accepts Canadian dollars and you want to invest using Canadian dollars - start the investment process as normal and when you get to the wallet balance section click the toggle button on the top right to flip the wallet from your USD wallet to your CAD wallet.
On the CAD wallet you will see a second box so that you can choose how much USD you would like to invest and how much it will cost in CAD from your CAD wallet.
FAQ
Q: What currency will my dividends be paid in?
A: Dividends will be payable in USD and required a USD bank account to withdraw
Q: Are there any US tax implications for this investment?
A: As always, addy recommends speaking to an expert. Usually US investments are into a Canadian legal structure so that Canadian investors don't have to worry about United States taxes but you should always consult a professional for more details about a specific issuance and its tax implications.