Jumping from Nightfall to Prophesies

I am probably be going to be asked why I wish to do this but I will ask the question anyway. is it possible to enter the Prophesies game when I built my charactor in Nightfall ? I do have all four parts of GW, prophesies,Factions, NightFall, And IOTN.
I first started with my earliest character in prophesies and added the rest on from there. I am now on my eighth character but only the first two were started in prophesies I started most from Factions & Nightfall which allowed movement into the later GW games but I cannot seem to find any way to to enter the Ascalon area as I am getting totally frustrated with the other games as I have now built a character I enjoy playing with but although I have not completed all of the Missions / quests in any of them I am unable to find the "triggers" or complete the more difficult quests to continue.

The missions in IOTN I have not yet been able to complete are "The Heart of Shiverpeaks" as I have not been able to defeat Cyndr the mountain heart. Or "The Anvil of Dragrimmer" and unable to defeat "the Remnant of Antiquities" or the "Veilded threat " Mission as I cannot even get the the third level to defeat Rragar in his menagerie. or the "Against the Destroyers" mission to seek out Jalis Iron Hammer" I suspect that if I was /am able to complete those missions others would open up for me but I have not been able to do so thus far.

Also in Nightfall I have not been able to complete / win the Dzagonur Bastion Mission and there does not seem to be many more to complete I have completed 15/17 of Nightfall but no others are opening up for me to play.also at some stage during my game play I chose to play on with the "Master of Whispers" and not "Margrid the sly" and consequently any missions concerning her are now closed to me.

I have through frustration explored my way around the maps and entered the towns / outposts just making my way to them but when I finally get there the quests / missions which I was able to access through other characters are not forthcoming, also I am unable to remember how to gain the entrance to the mission in the "Grand Court of Sebelkah" again I have played that mission with another character but cannot now find entry into it with this one.
Any advice on any of the above subjects would be very welcome as I enjoy playing the game so much, but the only missions now left open to me are very frustrating as I get fed up with getting my butt regularly kicked in trying to complete the above missions.

To answer your initial question, it is possible, once you reach the Consulate Docks you will have the option to go to Prophecies (Lion's Arch) or Factions (Kaineng Center).

Sounds to me like you need a Guild to group along with, so that you can coordinate and take those tougher challenges down. Or, take advantage of the PUGs and Grouping Forum, because a lot of other people have trouble there as well!

Just in general, the wiki gives great advice for overcoming many of these challenges, and gives you the resources to find counters to the rest.