However, today I wondered, is it somehow to possible to override the dependency requirements?
But in this case, the issue is really that the feature requires the "regular" version and Tycho won't change the feature. So even if you manage to hack the dependency resolution at build time, this resolution error you see now in Tycho would still be likely to happen later for people trying to install...
So in this case, you'll need to tweak the org.eclipse.ecf.filetransfer.httpclient45.feature (or most likely build a fork of it) so it doesn't require this specific version of the bundle, but can use any version. This is usually possible by replacing a direct <plugin> element (which mandates specific version) by a `<requires><import plugin="org.apache.httpcomponents.httpclient"/></requires>` which results in more open version range and would pick anyone that matches.
If you add the fixed bundle build and the "relaxed" feature build to your build, they should be used by the product instead of the ones from upstream, and it should then work; unless there is something else that requires the specific version of feature or bundle.