You can add a DIY ISA Portfolio to your existing InvestEngine ISA by following the steps below:
My dashboard-> ‘Add new portfolio’-> ‘Create DIY portfolio’-> ‘ISA’-> 'Continue'-> select ETF(s)->'Add to portfolio'-> 'Continue'-> 'Set portfolio weights'-> 'Review and continue'->'Save and continue'->Read and agree to the ISA declaration-> 'Continue'.
Once your new DIY ISA Portfolio is available via your dashboard you can add cash to it by arranging a transfer from your bank account — assuming you have yet to contribute your full £20,000 ISA allowance for this tax year — and get investing.
However, if you want to fund your new DIY ISA Portfolio from your existing Managed Portfolio, you would need to sell down all or part of your holdings in your Managed portfolio. Please note: you must ensure you do not withdraw any cash monies if you have a Managed ISA portfolio or from your ISA Account cash; if you do, you will loose that part of your ISA allowance.
Follow the instructions here on how to make internal transfers: Can I transfer funds between my portfolios?