Replace Text with other text

no, you cannot use PowerQuery with DAX custom column - but you can create the same column in PowerQuery itself (so it's no longer calcuated), you can do the PowerQuery equivalent of DAX LOOKUPVALUE by using Merge Queries functionality

And now what do I do?

you select your lookup table from the drop down, create a left join (your lookup key needs to be unique), and then expand new column (the one you were looking up)

this video is very helpful on working with PowerQuery, may be good to :

https://www.youtube.com/watch?v=KXxUDWwo0pg

this is what I have in my advanced editor:

let
Source = GoogleAnalytics.Accounts(),
#"" = Source{[Id=""]}[Data],
#"UA" = #""{[Id="UA"]}[Data],
#"148099323" = #"UA"{[Id=""]}[Data],
#"Added Items" = Cube.Transform(#"148099323",
{
{Cube.AddAndExpandDimensionColumn, "ga:eventAction", {"ga:eventAction"}, {"Event Action"}},
{Cube.AddAndExpandDimensionColumn, "ga:date", {"ga:date"}, {"Date"}},
{Cube.AddMeasureColumn, "Total Events", "ga:totalEvents"}
}),
#"Sorted Rows" = Table.Sort(#"Added Items",{{"Total Events", Order.Descending}}),
#"Filtered Rows" = Table.SelectRows(#"Sorted Rows", each [Event Action] = "Billing History - Loaded" or [Event Action] = "Billing - Loaded" or [Event Action] = "Change invoice delivery form - Finished" or [Event Action] = "Saldos - Clicked" or [Event Action] = "Reload balance - Clicked" or [Event Action] = "Transfer balance - Loaded" or [Event Action] = "Buy button - Clicked" or [Event Action] = "Consumos - Clicked" or [Event Action] = "SMS Eventos - Loaded" or [Event Action] = "Internet details - Loaded" or [Event Action] = "Call Eventos - Loaded" or [Event Action] = "Other transactions - Loaded" or [Event Action] = "Buzón de Voz - Deactivated" or [Event Action] = "chat aquí - Clicked" or [Event Action] = "My information - Loaded" or [Event Action] = "Go cnfiguration Smartapps - Clicked" or [Event Action] = "View contract - Clicked" or [Event Action] = "Billing history - Loaded" or [Event Action] = "Billing - Loaded" or [Event Action] = "Download invoice PDF - Clicked" or [Event Action] = "Change invoice delivery form - Finished" or [Event Action] = "Add card form - Finished" or [Event Action] = "Internet Service - Viewed" or [Event Action] = "Change password wifi form - Finished" or [Event Action] = "Change name network form - Finished" or [Event Action] = "Phone Service - Viewed" or [Event Action] = "Consumption history - Loaded" or [Event Action] = "Television Service - Viewed" or [Event Action] = "zendesk-chat - Starts" or [Event Action] = "Pay invoice - Clicked" or [Event Action] = "Payment form - Finished")
in
#"Filtered Rows"