why do you think a simple 6 to 8 pin adapter would fail to work as it has no moving parts and is simply changing a 6 pin connector to a 8 pin connector unless you buy a no name cheapo adapter from ebay that uses the wrong gauge wire there is nothing to go wrong
this forum has a "SEARCH" feature please use it and read all of the numerous posts, many of which directly pertain to your question
a more important question is the power supply,..... there is the optional 700 watt supply (with the two 6 pin gpu connectors)
and the base 525 watt unit (one GPU connector) which is not suitable for a 2080 much less a 2080 TI which draws 250 watts
https://www8.hp.com/h20195/v2/getpdf.aspx/c04400038.pdf
looking at the HP z440 quickspecs, no HP approved card is drawing over 100 watts, so i would caution you not to install a card that draws over 1.5 times the wattage of a HP highend approved card
you can however replace the power supply with a AXT supply that can support the 2080 TI power draw using a adapter