Ideally a vent hose should be as short as possible. If you want to extend the length of a vent hose on a vented tumble dryer you should be aware that vent hoses are designed to be a specific length by the manufacturer. This length is likely to be the maximum length that the dryer can be vented without potential problems. It will be technically possible to somehow connect two vent hoses together or otherwise extend its length but the problem is, will the dryer be able to pump the hot moist air right to the end of the extended hose? If it can’t, the dryer could at best be inefficient and at worst overheat.
As far as I’m aware manufacturers don’t make vent hose extension kits. It might be possible to extend one very slightly and get away with it but the dryer’s exhaust fan pumping out the hot moist air is not particularly powerful and may struggle to get the air efficiently out through an extended hose.
There will be a danger of condensation or even water accumulating inside (this can happen even with the original length), especially if there are bends and kinks in the hose. This will cause fluff and lint to stick to the inside of the hose restricting its diameter.
I haven’t had time to extensively research this issue so this advice is not necessarily definitive. I did find that in the US and Canada there appears to be a potential solution in that they have available an optional dryer vent booster fan, which can help take the moist air much further. If your vented dryer has to be installed far away from an outside wall or suitable window you might try searching for a dryer vent booster fan. It might be better to contact the manufacturer and ask their advice.
I have had a dryer with a 7 metre extra pipe ( rigid plastic 4″ pipe) approx 2 metre up then 5 metre gradual slope in roof space to external wall. The flexi pipe does accumulate additional fluff but quite easily cleaned. In 14 years I have “swept” the fixed pipework once, when the initial dryer gave up after 10 years. I simply pulled a large duster through the pipe. There was remarkably little accumulation. Have to say that the creeper on the outside wall seems to thrive with the moist warm air that it receives!