You can join RJ11 male-male with an RJ45 coupler ;
RJ45 coupler - example here
I would suggest the Cat5e version at the same price as, although it doesn't state, I would think it is shielded. You need the inline version.
Perhaps not the most elegant solution but it works. Alternatively you could invest in an RJ11 crimper, a couple of RJ11 ends and a piece of RJ11 type flat cable. At £3.99 vs about £25 I know which method I would choose !
Would just advise to route a reasonable distance (12 inches)away from power/lighting cables and fluorescent lighting if it's going via the attic.
Hope that helps.