When a linktrigger is currently imported or exported, it is defined as follows:

<linktrigger>
<triggerid>
<host>stub-nik-226</host>
<description>Egress traffic 226.202</description>
<expression>(

{stub-nik-226:lagtotal.egress.226.202.last(#30)}

> 2000000000) & (

{stub-nik-226:lagtotal.egress.226.202.last(0)}

<

{stub-nik-226:lagtotal.egress.226.202.prev(0)}

*0.5 |

{stub-nik-226:lagtotal.egress.226.202.last(0)}

<

{stub-nik-226:lagtotal.egress.226.202.last(#5)}

*0.5 |

{stub-nik-226:lagtotal.egress.226.202.last(0)}

<

{stub-nik-226:lagtotal.egress.226.202.last(#15)}

*0.5 |

{stub-nik-226:lagtotal.egress.226.202.last(0)}

<

{stub-nik-226:lagtotal.egress.226.202.last(#30)}

*0.5)</expression>
</triggerid>
<drawtype>0</drawtype>
<color>DD0000</color>
</linktrigger>

As you can see, generating such an expression again for a linktrigger would be quite difficult. In my opinion, there should be a way to just point to the already existing trigger without specifying the entire expression again.